The present invention relates to a polarization-independent optical pulse characterization instrument which analyzes time and frequency (wavelength) properties of an optical pulse in an arbitrary polarized state.
As a technology for analyzing time and frequency properties of an optical pulse by measuring a spectrogram which is a function of delay time and a frequency (or a wavelength), a method called frequency-resolved optical gating (FROG) has been developed. Change with time or change with frequency (wavelength) in intensity and phase of an optical pulse to be measured can be obtained from a spectrogram. This technique is reported in the following documents: Review of Scientific Instruments, Vol. 68, No. 9, pp. 3277–3295, 1997; Physica Status Solidi (b) Vol. 206, pp. 119–124, 1998; and IEEE Journal of Quantum Electronics, Vol. 35, No. 4, pp. 421–431, 1999.
As a technology for characterizing a feeble ultrashort optical pulse with high sensitivity and high time resolution at the time of optical fiber transmission, FROG to which two-photon absorption in a semiconductor is applied as an optical gate has been developed. This technique is reported in Optics Express, Vol. 7, pp. 135–140, 2000. With this technique, the following method for measuring a spectrogram is disclosed: a probe optical pulse and a gate optical pulse in a linear polarization state, in which both of the optical pulses are orthogonal to each other, are colinearly entered in a two-photon absorption medium; and thereby a spectrogram is measured as a function of delay time between the gate optical pulse and the probe optical pulse, and as a function of a frequency or a wavelength.
In a long-distance optical fiber transmission system, it is expected that polarization mixing and polarization mode dispersion caused by double refraction in an optical fiber will exert a serious influence upon signal degradation. Therefore, characterization of the influence of polarization mixing and polarization mode dispersion in the optical fiber exerted upon ultrashort optical pulse transmission is indispensable for enhancing performance of the long-distance optical fiber transmission system which uses an ultrashort optical pulse. However, in the conventional method for characterizing an optical pulse using FROG which applies two-photon absorption in a semiconductor as an optical gate, an optical pulse to be measured and a gate optical pulse must always be in a linear polarization state. Therefore, the conventional method cannot be applied to an arbitrary polarized optical pulse. It is impossible to correctly characterize a randomly polarized optical pulse, and an optical pulse, a waveform of which is distorted, which are caused by polarization mixing and polarization mode dispersion.
An object of the present invention is to provide an optical transmission system, a signal error rate of which is low. Another object of the present invention is to provide an instrument of optical pulse characterization which is useful to provide such an optical system.